Problem: Solve for $n$ : $n + 13 = 4$
Solution: Subtract $13$ from both sides: $ \begin{eqnarray} \\ n + 13 &=& 4 \\ \\ {-13} && {-13} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{13 + n} &\hphantom{=}& \hphantom{4} \\ n &=& 4 {- 13} \\ \end{eqnarray} $ Simplify. $ n = -9$